The location and identification of ground faults on ungrounded direct current electrical systems which are used extensively in electric utility stations and substations have always presented difficulties to the lineman or technician faced with the problem. Such ungrounded electrical systems are normally energized by a large storage battery of 120-240 volts which supplies circuit breakers, motor operated devices, electronic apparatus and many other items. Also, connected to these direct current electric systems are ground fault relays which protect the generating substation or distribution equipment, such as generators, transformers, and overhead/underground lines. It is therefore extremely important that these ungrounded direct current electrical systems remain free of low resistance ground faults (generally less than 100,000 ohms) in order to be able to operate equipment connected to the system and to avoid inadvertent disconnection of such equipment as generators, transformers, or electric distribution lines due to false relay operation.
Because the direct current system is by design ungrounded, a single ground fault appearing in such a system will not affect its operation. However, problems and troubles do arise when a second ground appears at a location, which in effect, results in and causes a phantom circuit through ground that may erroneously energize a device, such as a tripping coil for a circuit breaker, which in turn would open the circuit for a critical electric generator.
Additionally, from time to time ground faults will appear in these direct current systems and are due to such causes as moisture, deterioration of insulation or foreign non-insulating materials bridging the "live" conductor to a ground point. Generally, the ground faults first appear as very high resistance faults (generally greater than 100,000 ohms), but as time passes, the fault condition deteriorates to a point whereby a sensitive instrument or fault detector permanently connected to the system indicates that a ground, or multiple grounds, exists which should be located and removed before serious trouble or problems are encountered.
There are several problems which the lineman or technician encounters when locating sources of trouble on these ungrounded direct current systems. First, even when a ground is indicated by the ground detector, it is often very difficult to take the system out of service to locate the trouble. The most common prior art method of locating ground faults utilizes a technique of removing a small section of the system from service; one at a time, by opening switches, and then determining if the signal at the ground fault indicator has cleared. However, these methods have three disadvantages: (1) a portion of the direct current system must be removed from service, and spare equipment either substituted for it or the function abandoned for the time used in locating the fault; (2) because of the sensitivity of these circuits, switching transients may sometimes result in the triggering of relays and unwanted operations of equipment; and (3) if multiple ground faults are present, the sectionalizing procedure is unsatisfactory and will not succeed.
Suggested improvements over the sectionalizing fault location procedure is the use of tracer current methods, a number of which are described in U.S. Pat. Nos. 2,291,533, 2,300,771, 2,651,021 and 4,129,825. In addition to these prior art patents, a technical paper from the Japan IERE Council entitled "Faulty Control Device for D.C. Control Circuit", dated March 1978, describes a low frequency tracer current apparatus for locating grounds on ungrounded battery supplied direct current control systems. In each of these prior art disclosures, the problem of capacitive carry-over current beyond the point of fault is mentioned as a problem and each prior art apparatus utilizes some type of system or equipment in attempting to minimize this effect. Capacitive carry-over current occurs when the capacitive impedance of the cable beyond the location of the fault is sufficiently low in comparison to the impedance of the fault so that a significant tracer current signal can be detected beyond the point of fault. Accordingly, such disclosures attempt to overcome this problem by employing a relatively low frequency for the tracer current so that the signal through the fault is much greater than that of the charging current beyond the fault.
In U.S. Pat. No. 2,300,771, the charging current is cancelled by means of applying an identical signal 180 degrees out-of-phase to a parallel identical non-faulted conductor which is in close proximity to the faulted conductor. However, when utilizing such a technique, it is necessary to have the magnetic pickup of the detecting device encircle both of these conductors at the same time for the charging current signals to cancel. Also, the conductors must not be a pair which is serving a load(s) for which the fault must be found while the system remains in operation and for which a parallel, identical, unfaulted and unrelated conductor is available. Thus, such a technique has been unacceptable to the power generation industry.
Additionally, all known prior art tracer current methods utilize conventional magnetic pickup devices which are susceptible to stray electric and magnetic fields external to that produced by the tracer current in the faulted conductor. This is particularly true in electric generating stations and substations where very large magnetic and electric fields are ubiquitous. Thus, a high frequency tracer current results in undue charging carry-over current problems while a low frequency tracer current is difficult to detect and efficiently filter before measuring or amplifying the signal. The 32 hertz signal utilized by the prior art device, described in the Japan IERE paper, is too close to the first sub-harmonic of the 60 hertz power frequency used in the United States for efficient filtering of unwanted stray signals and the 10 hertz signal utilized in U.S. Pat. No. 2,651,021 is too low for efficient detection of small tracer currents through high resistance faults. Accordingly, such suggested tracer techniques have not been acceptable for locating grounds on energized ungrounded direct current systems.